Cryptosporidiosis is a top ten cause of infant diarrhea in low and middle-income countries (LMICs), as well as malnutrition and impaired neurocognitive development, leading to substantial yearly morbidity, mortality and lost disability-adjusted life years. A vaccine for cryptosporidiosis is not available, but would be appealing for prevention of cryptosporidiosis in children in LMICs where the burden of infection is high. Studies done by Dr. William A. Petri and Dr. Carol Gilchrist at University of Virginia have uncovered IgA correlates of protection against Cryptosporidium infection, which may be expoited for development of a vaccine. However, the cryptosporidial proteins targeted by protective IgA antibodies remain to be discovered. Antigen Discovery, Inc (ADI) of Irvine, California has developed a pilot scale Cryptosporidium protein microarray, which can be expanded and used to screen antibody responses against the cryptosporidial proteome. A proteome-scale platform for antibody immune-profiling has never before been available to the cryptosporidiosis research community, and this technology has the power to rapidly advance our understanding of the protective immune response directed to cryptosporidial proteins. A pan-proteome Cryptosporidium microarray will be developed to measure specific anti-cryptosporidial IgA levels in two mother-infant birth cohorts with years of follow-up and detailed characterization of clinical endpoints for diarrhea, including disease attributable detection of many diarrhea-causing pathogens, including Cryptosporidium spp. The aim of the study is to identify the IgA in maternal breast milk ingested by infants that correlates with reduced risk of subsequent Cryptosporidium infection and diarrhea. The array results and most promising vaccine candidates will be validated by producing the proteins in a eukaryotic expression system to include post-translational modifications and correct tertiary structure, which will be used to develop ELISA and Western blot assays for testing breast milk samples. The success of this study will present an opportunity for development of maternal vaccines to prevent cryptosporidiosis in infants during the vulnerable period of the first months of life. We postulate that protective antibodies identified in breast milk may also be protective when produced by the infant mucosal immune response, thus presenting the opportunity for development of a pediatric vaccine for continued protection. Protective antibodies may also be developed as therapeutic antibodies that can be given to treat persistent infection in immunocompromised patients, such as those with HIV/AIDS. We expect to identify 240-400 immunoreactive Cryptosporidium proteins, at least 3 of which will have increased IgA levels in the breast milk of mothers with infants that had lower incidence rates or protection from diarrhea, as well as fecal IgA from the infants? adaptive immunity. This grant application addresses the significant problem of cryptosporidiosis in children by laying the foundation for a vaccine through the study of specific mucosal IgA responses associated with protection.